1. Field
The present disclosure generally relates to optical networks. More specifically, the present disclosure relates to a multi-chip module (MCM) that includes integrated circuits that communicate via a butterfly optical network without optical-waveguide crossings in a single layer of the butterfly optical network.
2. Related Art
Wavelength division multiplexing (WDM), which allows a single optical connection to carry multiple optical links or channels, can be used to provide: very high bit rates, very high bandwidth densities and very low power consumption. As a consequence, researchers are investigating the use of WDM to facilitate inter-chip communication. For example, in one proposed architecture chips in an array (which is sometimes referred to as a ‘macrochip,’ a multi-chip module or an MCM, and the chips are sometimes referred to based on the locations or ‘sites’ where they are placed in the array) are coupled together by an optical network that includes optical interconnects (such as silicon optical waveguides).
In order to use photonic technology in interconnect applications, an efficient design is desired for the optical network. In particular, the optical network ideally provides: a high total peak bandwidth; a high bandwidth for each logical connection between any two sites in the array; low arbitration and connection setup overheads; low power consumption; and bandwidth reconfigurability.
A variety of optical network topologies having different characteristics and contention scenarios have been proposed to address these challenges in interconnect applications. In general, the topology for the optical network that couples the chips is dependent on application demands and on potential future improvements in optical technology. For example, some applications only require processes on each chip to communicate with one other chip or a small number of other chips. In these cases, an optical network that allows the chips to use their full optical communication bandwidth to communicate with one other chip may provide the most efficient use of optical-network communication bandwidth, and if the complexity of the optical link used to support this communication technique is not too high, then this optical network may also be the most energy-efficient choice.
A variety of such optical networks (such as: crossbar, butterfly and Clos topologies) have been designed and implemented in the electronic domain. However, when implemented in silicon photonics, such topologies often have complicated optical-waveguide routing which can result in long optical-waveguide lengths and optical-waveguide crossings. The latter are problematic because in-plane optical-waveguide crossings introduce both crosstalk and signal loss. For example, a single-plane implementation of a 16-node butterfly optical network may include a worst-case optical link with 207 optical-waveguide crossings between switch stages. Using current technology, each optical-waveguide crossing may result in approximately 0.1 dB of signal loss, which may result in 20 dB total per-optical-link signal loss. In the absence of a significant breakthrough in optical-waveguide crossing design that achieves 0.0007 dB signal loss per optical-waveguide crossing, this single-plane design is not feasible.
Hence, what is needed is an MCM with an optical network that does not suffer from the above-described problems.